Conventionally, an image forming apparatus such as an MFP (Multi Function Peripheral) has been used to form an image on a sheet. In addition, an image erasing apparatus has been used to erase an image formed on the sheet with an erasable coloring agent such as a toner containing leuco dye when the image is not necessary anymore, so that the sheet can be reused.
Because such an erasable coloring agent is decolorized when heated to a high temperature, the sheet can be reused after an image on a sheet is heated and erased by the image erasing apparatus. In such an image erasing apparatus, a press roller and a heat source are oppositely arranged with each other across a sheet conveyance path, and a sheet is conveyed between the press roller and the heat source such that an erasable coloring agent on the sheet is decolorized by the heating. The aforementioned erasing of the image formed on a sheet is hereinafter referred to as ‘color erasing.’ The reuse of sheets saves sheets and is beneficial to the environment.
Further, the conventional image erasing apparatus determines whether or not each of the sheets is reusable after the image printed on the sheet with an erasable coloring agent is processed by an erasing section, and sorts out the sheets by conveying reusable sheets to a reuse cassette and non-reusable sheets to a reject cassette according to the result of the determination. The reusability is determined according to whether or not there is a residual image and the state (deformed, damaged, stained, and the like) of the sheet.
In addition, as there is a practical limit on the number of times sheet can be reused, a sheet is conveyed to the reject cassette as a non-reusable sheet if it has been reused for N times.
However, the erasable toner is not necessary to print an image on the sheet that will not be reused any more as the erasing will not be carried out. The use of the erasable toner in such a printing leads to an unnecessary increase in cost as the erasable toner is generally more expensive than ordinary toners.